1. Field of the Invention
The present invention is concerned with piezoelectric pressure control valves. More specifically, the invention relates to a valve used in conjunction with a system, such as an anti-lock braking system, which requires high frequency actuation capabilities.
2. Background Art
The piezoelectric effect was discovered by Jacques and Pierre Curie in 1880. They found that certain materials deform when exposed to an electrical field. This has become known as the inverse piezoelectric effect. The effect is practically linear, i.e., the deformation varies directly with the applied potential difference. It is also direction-dependent, so that if the material is exposed to an electric field, it will experience an elastic strain which causes its length to increase or decrease according to the polarity of the field. This behavior is manifest in such materials as piezoelectric ceramics, which are hard, chemically inert, and completely insensitive to humidity and other atmospheric influences.
As an example of harnessing piezoelectric phenomena, U.S. Pat. No. 4,690,465, which issued on Sep. 1, 1987, discloses an anti-skid hydraulic pressure modulator for a vehicular hydraulic braking system. The system includes a piezoelectrically operated pressure modulator, including a passage which is opened and closed by a piezoelectric actuator. The piezoelectric element expands and contracts almost instantaneously in response to voltage application and voltage drop. However, in such systems, responsiveness is limited in operation by a shut-off valve being positioned either in an "on" or "off" position. Such systems do not exhibit a continuously controlled modulation.
An anti-lock brake system is one example of a device which may be controlled by a pressure control valve. Illustrative of other such devices are vehicular transmission systems, engine intake and outlet valves, and suspension systems. In the past, the effectiveness of such devices has been limited by the inherent sluggishness or delay with which fluid flow, for example, changes in response to a sensed condition (such as applied brake pressure).
A member component of most piezoelectric pressure control valves is a movable spool and seat made of a silicious material such as silicon. Electrically activated, normally closed diaphragm valves have been fabricated using heated bi-metallic diaphragms to provide the operating force. Such valves have been designed to provide proportional control of flows at various input pressures. The valves are fabricated using silicon micromachining techniques. In an article entitled "Electrically Activated, Normally-Closed Diaphragm Valves" by H. Jerman (91CH2817-5/91 IEEE), the author observes that conventional valves for flow control have typically used magnetic actuation in the form of solenoids or motors to drive spool valves. The reference notes that valve actuation is possible using piezoelectric drivers, but the properties of such materials produce high forces with a very small deflection for button-type actuators. The reference states that such a driver has been reported as a valve actuator, but the complicated assembly and high voltage operation is said to be unattractive for many commercial applications.
Commonly assigned U.S. Pat. No. 4,768,751 which issued on Sep. 6, 1988 discloses a silicon micromachined non-elastic flow valve. That reference discloses a valve assembly for controlling fluid flow, including an actuator and a fluid. The actuator separates the nozzle plate from a valve plate, thereby permitting fluid flow. Also disclosed is a spring means for biasing the nozzle plate into a closed position to arrest fluid flow.
Against this background, there remains an unsatisfied need for low cost pressure control valves which can be made in large quantities and which exhibit a higher frequency response than those presently known, wherein there is a continuous modulation of flow output in response to an input signal which is communicated to the valve.